Osteoporosis is a symptom characterized by a decrease in the density of bone structures due to a lack of calcium, for instance. One method of diagnosing osteoporosis having been heretofore proposed is to quantitatively determine a bone characteristic (e.g., bone salt density) by passing ultrasonic waves through a bone and measuring the propagation speed of sound (SOS) in the bone. An ultrasonic examination of this kind is usually performed on a heel of the foot where the thickness of soft tissue is small.
A conventional apparatus (which is referred to as an ultrasonic bone-salt determination apparatus) for measuring the bone characteristics in the aforementioned fashion is movably mounted on a platform and, thus, makes use of a pair of movable parts with an ultrasound generator and an ultrasound detector installed face to face with each other individually on inside walls of the movable parts. A human heel is placed between the movable parts to interrupt a straight path between the ultrasound generator and detector, and the movable parts are repositioned such that the ultrasound generator and detector come in close contact with the heel in the air to permit propagation of ultrasonic waves. If ultrasound is emitted from the generator in this condition, it propagates at a speed that is dependent upon the bone salt content of the heel bone when passing through it. Thus, it is possible to measure a quantity corresponding to the bone salt content of the heel bone, which is the body part to be examined, by determining the speed of sound (SOS) of the ultrasound with the detector.
Specifically, the speed of sound (SOS) (SOS=L/T (m/s)) in the body part to be examined (heel) is derived from the distance L between the ultrasound generator and detector and the travel time T needed for a sound signal to propagate through the heel obtained by the ultrasound to pass through the body part to be examined (heel) as described above. The speed of sound (SOS) is converted into the corresponding bone salt content of the body part to be examined (heel) and, then, a judgment is made concerning the presence or absence of osteoporosis mentioned above. The apparatus intended for use in the above-described diagnosis is referred to as a dry-type ultrasonic diagnostic apparatus.
A pair of movable parts are used in the aforementioned dry-type ultrasonic diagnostic apparatus due to the need for holding the ultrasound generator and detector in close contact with the body part to be examined (heel). This arrangement, however, has made it essential to measure the aforesaid distance L (i.e., the distance L between the ultrasound generator and detector) each time a measurement is made by passing the ultrasound through the body part to be examined (heel). Further, it is necessary to move the individual movable parts by using a robust, precision mechanism and prepare an electric signal processing system featuring good linearity and high resolution for measuring the distance L with a high degree of accuracy. It has, however, been difficult to measure it with good accuracy, which results in an increase in the cost of the apparatus itself. Since it has been difficult to measure the distance L between the ultrasound generator and detector with good accuracy as stated above, the propagation speed of sound (SOS) determined would vary from one measurement to another or contain a significant error, rendering measurement results unreliable.
A previously proposed solution to the aforementioned problems is found in an ultrasonic diagnostic apparatus described in Japanese Unexamined Patent Publication No. 5-228148. This kind of apparatus employs a frame associated with an ultrasound generator and detector which are attached to the inside of the frame as well as a liquid-filling bag (bolus) provided within the frame. The bolus is filled with a matching liquid so that the body part to be examined (heel) is surrounded by the matching liquid without allowing any gaps around the body part. If ultrasound is emitted from the generator in this condition, it propagates at a speed dependent upon the bone salt content of the heel bone when passing through it. Thus, it is possible to measure a quantity corresponding to the bone salt content of the heel bone, which is the body part to be examined, by determining the speed of sound (SOS) of the ultrasound with the detector, and thereby make a judgment about the presence or absence of osteoporosis. The apparatus designed to diagnose in this manner is hereinafter referred to as a bolus-type ultrasonic diagnostic apparatus.
The distance L between the ultrasound generator and detector is fixed by attaching them to the inside of the frame as described above. With this arrangement, it becomes possible to eliminate variations in the propagation speed of sound (SOS) obtained in successive measurements and large errors in measurement results which would occur in the aforementioned dry-type ultrasonic diagnostic apparatus.
In the bolus-type ultrasonic diagnostic apparatus, however, propagation of the ultrasound up to the body part to be examined (heel) is ensured by filling the liquid-filling bag (bolus) with the matching liquid and thereby increasing the volume of the liquid-filling bag. For this reason, elastic fatigue of the liquid-filling bag (bolus) increases in proportion to the number of times it is used for measurement and diagnosis (i.e., the number of times the volume of the liquid-filling bag is increased and decreased), and this would potentially develop a problem concerning its durability. Especially because the liquid-filling bag is required to be able to wrap around the body part to be examined (heel) regardless of its physical size, any material used for making the liquid-filling bag must be flexible enough to provide a great dimensional coverage. Further, the material of the liquid-filling bag must be made thin enough to provide absolutely required ultrasound-transmitting properties.
Therefore, a material which would cause deterioration of the liquid-filling bag (bolus) and its fracture in a short time is not only useless but would develop such an accident that the matching liquid spilled out of the liquid-filling bag (bolus) adheres to the body part to be examined (heel). This will not pose any problem to the human heel if the matching liquid is water. Potential danger exists, however, because the matching liquid could contain chemicals, other than water, in certain cases for altering temperature response of ultrasound propagation velocity, or for preventing deterioration of the material, for instance.
Since the liquid-filling bag (bolus) is formed of a flexible and tough material, it may develop wrinkles as a matter of course when it is in service. Formation of such wrinkles in a surface that comes into contact with the body part to be examined (heel) would result in extremely abnormal transmission and reflection of ultrasonic waves, making it impossible to perform any measurement.